The application of mechanical advantage to enable manually applied forces to be multiplied for accomplishing tasks requiring super human strength is well known. Levers and pulleys are commonly used to enable men to lift weights far exceeding the force which they are capable of directly applying.
Fluid driven pistons slidably disposed in cylinders have long been employed in the application of mechanical advantage to accomplish lifting tasks. Bottle jacks are examples of lever actuated fluid driven piston devices which enable a man to lift a vehicle weighing several tons off the ground.
The forces generated by fluid driven piston devices can be harnessed for use in applications other than lifting. For example, hand held hydraulic presses which are fluid driven piston devices that use mechanical advantage to multiply force are used by fire and police departments to separate two members after they are wedged between a door and a door frame to force the door open.
Conventional hand held hydraulic presses generally have a tubular reservoir to store the hydraulic fluid, a piston slidably mounted in a cylinder which may extend from the reservoir, a hose into which fluid from the reservoir is pumped from the cylinder by the action of the piston, and an operating device, e.g., a separable toe and foot door forcer, attached to the other end of the hose for being actuated by the fluid pumped under high pressure. In addition to being large and clumsy to use, conventional hand held hydraulic presses depend on gravity to keep the fluid in the reservoir and must, therefore, be operated in an upright position. If they are turned upside down, the fluid flows out of the reservoir and cannot be pumped into the cylinder to exert force against the piston.